The vmap() allocation failure is due to kernel virtual memory exhaustion, which is not an uncommon problem on Linux/x86 systems. The size of the kernel stacks is unrelated to this - if you're seeing cases in which the use of 4KB stacks vs. 8KB stacks breaks the NVIDIA driver, please report this separately. The NVIDIA Linux driver is designed to work with 4KB kernel stacks, and there are no known problems with 4KB configurations.